Informational data in typical liquid crystal displays (LCDs) is presented in the form of a matrix array of rows and columns of numerals or characters (i.e. pixels) which are generated by a number of segmented electrodes arranged in a matrix pattern. The segments are connected by individual leads to driving electronics which apply a voltage to the appropriate combination of segments and adjacent LC material in order to display the desired data and/or information by controlling the light transmitted through the liquid crystal material.
Contrast ratio is one of the most important attributes considered in determining the quality of both normally white (NW) and normally black (NB) liquid crystal displays. The contrast ratio in a normally white display is determined in low ambient conditions by dividing the "off-state" light transmission (high intensity white light) by the "on-state" or darkened transmitted intensity. For example, if the "off-state" transmission is 200 fL at a particular viewing angle and the "on-state" transmission is 5 fL at the same viewing angle, then the display's contrast ratio at that particular viewing angle is 40 or 40:1 for the particular "on-state" driving voltage utilized.
Accordingly, in normally white LCDs a significant factor adversely limiting the contrast ratio is the amount of light which leaks through the display in the darkened or "on-state." In a similar manner, in normally black displays, a significant factor limiting the contrast ratio achievable is the amount of light which leaks through the display in the darkened or "off-state." The higher and more uniform the contrast ratio of a particular display over a wide range of viewing angles, the better the LCD in most applications.
Normally black (NB) twisted nematic displays typically have better contrast ratio contour curves or characteristics then do their counterpart NW displays (i.e. the NB image can be seen better at large viewing angles). However, NB displays are much harder to manufacture than NW displays due to their high dependence on the cell gap or thickness "d" of the liquid crystal layer as well as on the temperature of the liquid crystal material itself. Accordingly, a long-felt need in the art has been the ability to construct a normally white display with high contrast ratios over a large range of viewing angles, rather than having to resort to the more difficult to manufacture NB displays to achieve these characteristics.
What is generally needed in NW displays is an optical compensating or retarding element(s), i.e. retardation film, which introduces a phase delay that restores the original polarization state of the light, thus allowing the light to be substantially blocked by the output polarizer in the "on-state." Optical compensating elements or retarders are known in the art and are disclosed, for example, in U.S. Pat. Nos. 5,184,236; 5,189,538; 5,406,396; 4,889,412; 5,344,916; 5,196,953; 5,138,474; and 5,071,997.
The disclosures of Ser. Nos. 08/255,971 and 08/167,652 (both incorporated herein by reference) in their respective "Background" sections illustrate and discuss contrast ratios and driving voltage versus intensity (fL) graphs of prior art NW displays, which are less than desirable. Prior art NW LCD viewing characteristics are problematic in that, for example, their contrast ratios are limited both horizontally and vertically (and often non-symmetric), and their gray level performance lacks consistency.
Gray level performance and the corresponding amount of inversion are also important in determining the quality of an LCD. Conventional active matrix liquid crystal displays typically utilize anywhere from about 8 to 64 different driving voltages. These different driving voltages are generally referred to as "gray level" voltages. The intensity of light transmitted through the pixel(s) or display depends upon the driving voltage utilized. Accordingly, conventional gray level voltages are used to generate dissimilar shades of color so as to create different colors when, for example, the shades are mixed with one another.
Preferably, the higher the driving voltage in a normally white display, the lower the intensity (fL) of light transmitted therethrough. The opposite is true in normally black displays. Thus, by utilizing multiple gray level driving voltages, one can manipulate either a NW or NB liquid crystal display to emit desired intensities and shades of light/color. A gray level V.sub.ON is generally known as any driving voltage greater than V.sub.th (threshold voltage) up to about 5 to 6.5 volts.
Gray level intensity in an LCD is dependent upon the display's driving voltage. It is desirable in NW displays to have an intensity versus driving voltage curve wherein the intensity of light emitted from the display or pixel continually and monotonically decreases as the driving voltage increases. In other words, it is desirable to have gray level performance in a NW pixel such that the intensity (fL) at 6.0 volts is less than that at 5.0 volts, which is in turn less than that at 4.0 volts, which is less than that at 3.0 volts, which is in turn less than that at 2.0 volts, etc. Such desired gray level curves across a wide range of view allows the intensity of light reaching viewers at different viewing angles to be easily and consistently controlled.
Ser. Nos. 08/255,971 and 08/167,652 discuss, in their respective "Background" sections, prior art NW LCDs with inversion problems (e.g. inversion humps, specifically their transmission versus driving voltage graphs). As discussed therein, inversion humps are generally undesirable. A theoretically perfect driving voltage versus intensity (fL) curve for an NW display would have a decreased intensity (fL) for each increase in gray level driving voltage at all viewing angles. In contrast to this, inversion humps represent increase(s) in intensity of radiation emitted from the LCD or light valve for each corresponding increase in gray level driving voltage. Accordingly, it would satisfy a long-felt need in the art if a normally white TN liquid crystal display could be provided with no or little inversion and improved contrast ratios over a wide range of viewing angles.
U.S. Pat. No. 5,344,916 discloses a liquid crystal display including retardation films having positive and negative retardation films. The negative retarders (or birefringent films) of the '916 patent have as a characteristic that n.sub.x =n.sub.y &gt;n.sub.z, this equation defining the term "negative." The "Z" direction or axis is perpendicular to the plane of the film, while the "X" and "Y" axes are parallel to the retardation film plane. Thus, the optical axes of the negative retardation films in the '916 patent are perpendicular to the film plane. It is noted that n.sub.x, n.sub.y, and n.sub.z are the respective indices of refraction.
Unfortunately, while use of the negative retardation films of the '916 patent improves contrast over some prior art LCDs, twisted nematic (TN) displays including same often still suffer from less than desirable contrast ratios at large viewing angles. Pointedly, the disclosure of the '916 patent does not appreciate, suggest, or disclose the use of negative and positive retarders together at specified values, ratios, and/or locations to even further improve viewing characteristics of an LCD as discussed below by the instant inventors.
U.S. Pat. No. 5,189,538 (and U.S. Pat. No. 5,138,747) discloses a super twisted nematic (STN) LCD including films having different birefringent values. Unfortunately, STN LCDs have no real correspondence or correlation to .apprxeq.90.degree. TN LCDs with regard to the behavior of the image due to retarders. In other words, teachings regarding retarders in STN devices (e.g. 270.degree. twist) generally little or have no relevance with regard to TN (.apprxeq.90.degree. twist) LCDs.
U.S. Pat. No. 4,889,412 discloses a LCD with electrically controlled birefringence (ECB) and negative anisotropy. Unfortunately, ECB displays do not use twisted nematic LC material as does the instant invention. Again, ECB display teachings are generally unrelated to TN (.apprxeq.90.degree. twist) displays with regard to retardation teachings and principles.
U.S. Pat. No. 5,291,323 discloses a liquid crystal display with "positive and negative compensating films each with its optical axis parallel to the surface." Unfortunately, the disclosure and teaching of the '323 patent are unrelated to TN displays such as those of the instant invention in that the '323 patent relates to supertwisted (e.g. 240.degree. twist) LCDs. It is also worth noting that the slow phase axis directions of certain phase plates in the '323 patent are parallel to the surface (i.e. plane defined by the layer).
The term "rear" when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing zones, and orientation films means that the described element is on the backlight side of the liquid crystal material, or in other words, on the side of the LC material opposite the viewer.
The term "front" when used herein but only as it is used to describe substrates, polarizers, electrodes, buffing zones and orientation films means that the described element is located on the viewer side of the liquid crystal material.
The LCDs and light valves herein include a liquid crystal material with a birefringence (.DELTA.n) of 0.084 at room temperature, Model No. ZLI-4718 obtained from Merck, unless specified otherwise.
The term "retardation value" as used herein means "d.multidot..DELTA.n" of the retardation film or plate, wherein "d" is the film thickness and ".DELTA.n" is the film birefringence.
The term "interior" when used herein to describe a surface or side of elements (or an element itself), means closest to the liquid crystal material.
The term "light valve" as used herein means a liquid crystal display including a rear linear polarizer, a rear transparent substrate, a rear continuous pixel electrode, a rear orientation film, an LC layer, a front orientation film, a front continuous pixel electrode, a front substrate, and a front polarizer (without the presence of color filters and driving active matrix circuitry such as TFTs). Such a light valve may also include a retardation film(s) disposed on either side of the LC layer as described with respect to each example and embodiment herein. In other words, a "light valve" may be referred to as one giant pixel without segmented electrodes.
It is apparent from the above that there exists a need in the art for a normally white liquid crystal display wherein the viewing zone of the display has both high contrast ratios and little or no inversion over a wide range of viewing angles.
This invention will now be described with respect to certain embodiments thereof, accompanied by certain illustrations wherein: